# Input impedance of a HPF

Started by November 18, 2011
```I sketched out a VCVS high pass filter with a cutoff of a few hertz, and
noticed that the input capacitor is 100nF. Will this cause problems for
the circuit driving it, or does the feedback in a VCVS "hide" the
capacitance and make the input impedance appear much stiffer?

I will be offline for a few days so I won't be able to answer questions.
Basically, I was thinking of driving it from an op amp, the bandwidth of
the circuit is about 200kHz but I want to chop off the stuff below 10Hz,
and the C could be say 33nF rather than 100nF; but it's going to be a
pretty large C here which rings warning bells to me. Perhaps there's
another way to implement a HPF that doesn't involve having to drive into
a large C?

Thanks in anticipation,

Nemo
```
```On Nov 18, 9:17=A0am, Nemo <N...@nocannedmeatproducts.nosirree> wrote:
> I sketched out a VCVS high pass filter with a cutoff of a few hertz, and
> noticed that the input capacitor is 100nF. Will this cause problems for
> the circuit driving it, or does the feedback in a VCVS "hide" the
> capacitance and make the input impedance appear much stiffer?
>
> I will be offline for a few days so I won't be able to answer questions.
> Basically, I was thinking of driving it from an op amp, the bandwidth of
> the circuit is about 200kHz but I want to chop off the stuff below 10Hz,
> and the C could be say 33nF rather than 100nF; but it's going to be a
> pretty large C here which rings warning bells to me. Perhaps there's
> another way to implement a HPF that doesn't involve having to drive into
> a large C?

Unless the 100nF cpacitor were connected directly to ground, it is
unlikely that it would give you a problem. In most filter circuits,
the input impedance is dominated by the resistive components on the
other side of the capacitor.

Run a simulation, and look at the current flowig through the capacitor
as a function of signal frequency.

--
Bill Sloman, Nijmegen

```
```On Nov 18, 12:17=A0am, Nemo <N...@nocannedmeatproducts.nosirree> wrote:
> I sketched out a VCVS high pass filter with a cutoff of a few hertz, and
> noticed that the input capacitor is 100nF. Will this cause problems for
> the circuit driving it, or does the feedback in a VCVS "hide" the
> capacitance and make the input impedance appear much stiffer?
>
> I will be offline for a few days so I won't be able to answer questions.
> Basically, I was thinking of driving it from an op amp, the bandwidth of
> the circuit is about 200kHz but I want to chop off the stuff below 10Hz,
> and the C could be say 33nF rather than 100nF; but it's going to be a
> pretty large C here which rings warning bells to me. Perhaps there's
> another way to implement a HPF that doesn't involve having to drive into
> a large C?
>
> Thanks in anticipation,
>
> Nemo

VCVC high pass filters mostly have a pronounced dip in input impedance
near the cutoff frequency.  They don't have the "near infinite" input
resistance
that the amplifier alone would have.  But as BIll suggested, running
some
simulations or doing the mathematical analysis would provide a better
clue.
This form also tends to have relatively poor sensitivities to
component variation.

```
```On Fri, 18 Nov 2011 08:17:54 +0000, Nemo wrote:

> I sketched out a VCVS high pass filter with a cutoff of a few hertz, and
> noticed that the input capacitor is 100nF. Will this cause problems for
> the circuit driving it, or does the feedback in a VCVS "hide" the
> capacitance and make the input impedance appear much stiffer?
>
> I will be offline for a few days so I won't be able to answer questions.
> Basically, I was thinking of driving it from an op amp, the bandwidth of
> the circuit is about 200kHz but I want to chop off the stuff below 10Hz,
> and the C could be say 33nF rather than 100nF; but it's going to be a
> pretty large C here which rings warning bells to me. Perhaps there's
> another way to implement a HPF that doesn't involve having to drive into
> a large C?

Well, without knowing what your driving circuit is it's hard to say.
Usually one gets _more_ concerned when one's input impedance goes down
rather than less, as one is usually making voltage-amplification circuits.

Analyze the circuit -- what answer do you get?

--
www.wescottdesign.com
```
```On Fri, 18 Nov 2011 12:51:41 -0600, Tim Wescott <tim@seemywebsite.com>
wrote:

>On Fri, 18 Nov 2011 08:17:54 +0000, Nemo wrote:
>
>> I sketched out a VCVS high pass filter with a cutoff of a few hertz, and
>> noticed that the input capacitor is 100nF. Will this cause problems for
>> the circuit driving it, or does the feedback in a VCVS "hide" the
>> capacitance and make the input impedance appear much stiffer?
>>
>> I will be offline for a few days so I won't be able to answer questions.
>> Basically, I was thinking of driving it from an op amp, the bandwidth of
>> the circuit is about 200kHz but I want to chop off the stuff below 10Hz,
>> and the C could be say 33nF rather than 100nF; but it's going to be a
>> pretty large C here which rings warning bells to me. Perhaps there's
>> another way to implement a HPF that doesn't involve having to drive into
>> a large C?
>
>Well, without knowing what your driving circuit is it's hard to say.
>Usually one gets _more_ concerned when one's input impedance goes down
>rather than less, as one is usually making voltage-amplification circuits.
>
>Analyze the circuit -- what answer do you get?

"Analyze"?  What do you think this is, a DESIGN group or something
?:-)

Pretty clear that the OP is describing a Sallen-Key (fancy wording for
crap) Filter.

...Jim Thompson
--
| James E.Thompson, CTO                            |    mens     |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

I love to cook with wine.     Sometimes I even put it in the food.
```
```On Nov 18, 3:17=A0am, Nemo <N...@nocannedmeatproducts.nosirree> wrote:
> I sketched out a VCVS high pass filter with a cutoff of a few hertz, and
> noticed that the input capacitor is 100nF. Will this cause problems for
> the circuit driving it, or does the feedback in a VCVS "hide" the
> capacitance and make the input impedance appear much stiffer?
>
> I will be offline for a few days so I won't be able to answer questions.
> Basically, I was thinking of driving it from an op amp, the bandwidth of
> the circuit is about 200kHz but I want to chop off the stuff below 10Hz,
> and the C could be say 33nF rather than 100nF; but it's going to be a
> pretty large C here which rings warning bells to me. Perhaps there's
> another way to implement a HPF that doesn't involve having to drive into
> a large C?
A large C isn=92t necessarily bad.  Opamps have no trouble driving an RC
highpass filter. (in my experience)

George H.
>
> Thanks in anticipation,
>
> Nemo

```
```On Fri, 18 Nov 2011 11:55:22 -0700, Jim Thompson wrote:

> On Fri, 18 Nov 2011 12:51:41 -0600, Tim Wescott <tim@seemywebsite.com>
> wrote:
>
>>On Fri, 18 Nov 2011 08:17:54 +0000, Nemo wrote:
>>
>>> I sketched out a VCVS high pass filter with a cutoff of a few hertz,
>>> and noticed that the input capacitor is 100nF. Will this cause
>>> problems for the circuit driving it, or does the feedback in a VCVS
>>> "hide" the capacitance and make the input impedance appear much
>>> stiffer?
>>>
>>> I will be offline for a few days so I won't be able to answer
>>> questions. Basically, I was thinking of driving it from an op amp, the
>>> bandwidth of the circuit is about 200kHz but I want to chop off the
>>> stuff below 10Hz, and the C could be say 33nF rather than 100nF; but
>>> it's going to be a pretty large C here which rings warning bells to
>>> me. Perhaps there's another way to implement a HPF that doesn't
>>> involve having to drive into a large C?
>>
>>Well, without knowing what your driving circuit is it's hard to say.
>>Usually one gets _more_ concerned when one's input impedance goes down
>>rather than less, as one is usually making voltage-amplification
>>circuits.
>>
>>Analyze the circuit -- what answer do you get?
>
> "Analyze"?  What do you think this is, a DESIGN group or something ?:-)
>
> Pretty clear that the OP is describing a Sallen-Key (fancy wording for
> crap) Filter.
>
>                                         ...Jim Thompson

I had to look up "VCVS" filter -- I ended up on Wikipedia's Sallen-Key
filter page.  Sallen-Key is a "degenerate" form of VCVS filter.
Presumably that means it hangs out in the wrong sorts of bars, and
propositions underage components to come to its apartment for "a little
fun".

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
```
```On Fri, 18 Nov 2011 15:55:59 -0600, Tim <tim@seemywebsite.please>
wrote:

>On Fri, 18 Nov 2011 11:55:22 -0700, Jim Thompson wrote:
>
>> On Fri, 18 Nov 2011 12:51:41 -0600, Tim Wescott <tim@seemywebsite.com>
>> wrote:
>>
>>>On Fri, 18 Nov 2011 08:17:54 +0000, Nemo wrote:
>>>
>>>> I sketched out a VCVS high pass filter with a cutoff of a few hertz,
>>>> and noticed that the input capacitor is 100nF. Will this cause
>>>> problems for the circuit driving it, or does the feedback in a VCVS
>>>> "hide" the capacitance and make the input impedance appear much
>>>> stiffer?
>>>>
>>>> I will be offline for a few days so I won't be able to answer
>>>> questions. Basically, I was thinking of driving it from an op amp, the
>>>> bandwidth of the circuit is about 200kHz but I want to chop off the
>>>> stuff below 10Hz, and the C could be say 33nF rather than 100nF; but
>>>> it's going to be a pretty large C here which rings warning bells to
>>>> me. Perhaps there's another way to implement a HPF that doesn't
>>>> involve having to drive into a large C?
>>>
>>>Well, without knowing what your driving circuit is it's hard to say.
>>>Usually one gets _more_ concerned when one's input impedance goes down
>>>rather than less, as one is usually making voltage-amplification
>>>circuits.
>>>
>>>Analyze the circuit -- what answer do you get?
>>
>> "Analyze"?  What do you think this is, a DESIGN group or something ?:-)
>>
>> Pretty clear that the OP is describing a Sallen-Key (fancy wording for
>> crap) Filter.
>>
>>                                         ...Jim Thompson
>
>I had to look up "VCVS" filter -- I ended up on Wikipedia's Sallen-Key
>filter page.  Sallen-Key is a "degenerate" form of VCVS filter.
>Presumably that means it hangs out in the wrong sorts of bars, and
>propositions underage components to come to its apartment for "a little
>fun".

Bwahahahahahaha!  Good one!

...Jim Thompson
--
| James E.Thompson, CTO                            |    mens     |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

I love to cook with wine.     Sometimes I even put it in the food.
```